There are different shapes of carabiners and different belay devices, and each one requires a different setup. There are also all sorts of fancy carabiners and belay devices, but we’re just going to focus on the most commonly used ones. Use locking carabiners for all belay devices.
Gri Gri – Use a D-shaped or modified D carabiner, ideally. This forces the gri gri along the spine in the right orientation, and prevents cross loading. If you are using a pear-shaped biner (aka HMS biner), make sure the gri gri is situation on the smallest side. This will prevent the carabiner from getting cross-loaded.
ATC – Use a pear shaped biner only. Also, make sure the ATC is on the top of the carabiner.
Munter – Use a pear shaped biner only and have the rope feeding through the widest side of the carabiner. Otherwise, you’ll find yourself having a lot of trouble flipping the rope from the braking to the feeding position and vice versa.
That’s it. It’s that simple. When in doubt, grab yourself an HMS carabiner, because it works for all 3 belay methods. Just make sure you orient your device accordingly.